The invention concerns a valve actuation mechanism for an internal combustion engine on an automotive vehicle. The invention also concerns an automotive vehicle, such as a truck, equipped with such a valve actuation mechanism.
Automotive vehicles, such as trucks, often rely on an engine brake function to slow down in order, for example, to reduce wear of the friction brake pads and to prevent overheating of the friction brakes, particularly on downward slopes. It is known to perform engine brake by acting on the amount of gas present in the cylinders of the engine in two distinct phases. In a first phase, when the pistons are near a bottom dead center, one injects exhaust gases into the chambers of the cylinders so as to slow down the pistons when they move towards their high level. This is done by slightly opening at least a valve connected to an exhaust manifold, while exhaust gases are prevented to be expelled from the exhaust pipe and thereby at a certain pressure above atmospheric pressure. In the second phase, the gases which are compressed by the piston are expelled from the chamber of the cylinder when the piston is at or near its top dead center position in order to prevent an acceleration of the piston under effect of volumic expansion of compressed gas. This is done by slightly opening a valve so as to expel gases from the cylinder. In most cases, the valve (or valves) which is (are) opened for the engine brake function is (are) a main exhaust valve. An engine brake system is described in document WO 9009514.
To perform these engine brake valves movements, also called engine brake valves lifts, the engine comprises, for each cylinder, a rocker acting, on the valves to open and close them. The rocker is acted upon by a rotating cam which has at least one lift sector to cause the lifting (opening) of the valve. If the valve is also an exhaust or an intake valve, the corresponding cam will comprise a main valve lift sector and one or several auxiliary valve lift sectors (also called main valve lift bump and auxiliary valve lift bump) When engine brake is needed, a cam follower surface of the rocker is moved in close contact with a cam of a camshaft moving the rocker so that the brake movements of the valve are obtained, when the cam follower interacts with the auxiliary valve lift sectors. In normal operating conditions of the engine, the valves should not perform these movements and the roller of the rocker is kept slightly remote from the cam so that the cam follower does not interact with the auxiliary valve lift sectors. The distance or clearance between the roller and the cam ensures that only the larger main lift sector on the cam, dedicated to the main exhaust event, causes an opening of the exhaust valve, but not one or several smaller auxiliary lift sectors dedicated to the engine brake function. This clearance is suppressed when engine brake is needed, by moving an activation piston of the rocker to make a close contact between the roller and the cam, so that engine brake dedicated lift sectors on the cam also cause an opening of the valve. An engine brake system having such valve actuation mechanism is described in WO-91/08381
In the case of a system where two valves are to be actuated, the piston can be in contact with the valves through a valve bridge.
When the engine brake valve opening(s) have been performed, a reset function is preferably to be performed. In other words, the activation piston needs to be moved towards its initial position in order to ensure that the valves are closed early enough in order to prevent extended valve lift overlap.
Engine brake systems generally comprise a control valve to direct pressurized control fluid pressure in a chamber adjacent to the piston to move the activation piston from its initial position to its engine brake actuation position. The control valve controls whether or not the engine brake function is activated. This control valve lets pressurized, control fluid flow, at a pressure of for example 2 to 5 bars, towards each rocker as long as the engine brake function is needed, which typically lasts several seconds or tens of seconds during which the engine and the cam shaft may perform several hundreds or thousands of complete revolutions. In some systems, a check valve is provided to prevent any fluid, flow out of the chamber, in some known systems, such as the one described in WO-91/08381, the check valve can nevertheless be forced to an open position, allowing the control fluid to escape the chamber when the engine brake is not needed. This is achieved when no control pressure is sent to the control valve. In known systems, there is only one control valve for several cylinders, so that it is not possible to use the control valve to empty the chamber to allow retraction of the piston, if such retraction is needed for a period of time inferior to one revolution of the camshaft.
It is known, for example from U.S. Pat. No. 6,253,730, to act on the check valve thanks to a stopper which is fixed to a housing, of the engine, so as to open the check valve and release fluid pressure in the chamber so that the piston may move towards its initial position, retracted. This technical solution does not insure a satisfying reliability.
It is desirable to provide a new valve actuation mechanism in which, when a specific operation function of the engine must be activated, the activation piston can be reset to its first position in a more efficient and reliable way than in the prior art.
To this end, an aspect of the invention concerns a valve actuation mechanism for an internal combustion engine on an automotive vehicle, comprising rockers moved by a camshaft, each rocker being adapted to exert a valve opening force on at least a portion of a valve opening actuator of each cylinder, via an activation piston, housed in a bore of the rocker, and movable with respect to the rocker under action of a fluid pressure raise in a chamber fluidly connected to the bore, from a first position to a second position, in which a cam follower of the rocker reads at least one auxiliary valve lift sector of a cam of the camshaft so as to perform an engine operation function, each rocker comprising a check valve adapted to control the fluid pressure raise in said chamber. This valve actuation mechanism is characterized in that it comprises, for each rocker, a stopper fast with a housing of the engine and adapted to exert, on a member of the rocker, a force for opening a valve independent from the check valve, adapted to reduce fluid pressure in the chamber, when the piston has to be moved from its second position to its first position.
Thanks to an aspect of the invention, the actuation piston is allowed to retract back to its first position by releasing the control fluid pressure in the chamber thanks to a valve independent from the check valve, and by a mechanical part acting directly on said valve independently for each rocker. This improves the reliability of the valve actuation mechanism.
According to fluffier aspect for the invention which are advantageous but not compulsory, such a valve actuation mechanism may incorporate one or several, of the following features:                The valve adapted to release fluid pressure from the chamber is a discharge valve adapted to allow fluid flow from the chamber to the outside of the rocker, wherein the piston comprises        a first element housed in the bore and movable in translation with respect to the rocker,        and a valve member housed in a portion of the first element and movable in translation with respect to the first element along a longitudinal axis of the piston, whereas the discharge valve is formed by a cooperation between the first element and the valve member, and wherein the force of the stopper is exerted on the first element.        The valve member is movable with respect to the first element between a first position, corresponding to a in the closed position of the discharge valve, in which a planar annular surface of the valve member is kept in abutment against a stop of the first element, and a pin of the valve member closes a passageway between the chamber and the hollow portion outside of the rocker.        A traction spring exerts a force between the valve member and the first element, tending to keep the discharge valve in its closed position.        The force exerted by the stopper on the first element is superior to the force exerted by the traction sprung.        In the closed position of a discharge valve, a planar annular surface of the valve member is kept in abutment against a stop of the first element.        In the opened position of the discharge valve, the valve member is offset with respect to the first element so that fluid can circulate between the chamber and the outside of the rocker.        The valve member comprises a contact surface adapted to exert the valve opening force on the valve opening actuator.        The valve member comprises at least one bleed passage adapted to allow passage of fluid from the hollow portion of the first element to the outside of the piston.        The valve member is housed in a hollow portion of the first element.        The valve adapted to reduce fluid pressure in the chamber is a reset valve, adapted to allow fluid flow from the chamber to the outside of the rocker, housed in a portion of the rocker distinct from the actuation piston, and wherein the force of the stopper is exerted on the reset valve        The reset valve comprises a ball spring-biased against a seat formed by a surface of the rocker, extending around a bleed passage connecting the chamber to the outside of the rocker, by a compression spring, and wherein the ball cooperates with the pin.        The force exerted by the stopper on the pin is superior to a force exerted by the compression spring on the ball.        The force exerted by the stopper on the member is variable.        The stopper comprises a main spring adapted, when deformed, to exert a compression force on a pushrod which is in contact with the movable member.        It is an exhaust valve actuation mechanism.        The activation piston activates an exhaust gases recirculation function when it is in its second position.        The activation piston activates an engine brake function when it is in its second position.        It is an intake valve actuation mechanism.        
The invention also concerns an automotive vehicle, such as a truck, comprising a valve actuation mechanism as mentioned here-above.